A substantial effort has been expended to extend the detectable wavelength of the III-V semiconductor photoemitters beyond the wavelength of about .lambda. = 1.24.mu., i.e., below 1.0 eV photon energy.
In general, there are basic limitations on the wavelength response of photoemitters, for example, the work function .epsilon.. In so-called "negative affinity" photoemitters, it has been shown that the "interfacial" barrier between the semiconductor and the activator is also an important limitation. In order to extend the detectable wavelength, such limitations must be overcome.
The most effective semiconductor photoemitters are P-type structures where the Fermi level more or less coincides with the valence band of the electrons in the bulk semiconductor. In the so-called multialkali photocathodes, the energy difference between the Fermi level at the semiconductor surface and the vacuum level, known as the work function .epsilon., is greater than the energy difference between the valence band and the conduction band of the electrons in the semiconductor, and the energy h.nu. of photons impinging on the semiconductor must be great enough to promote the electrons from the valence band to the higher energy conduction band and from the conduction band over the vacuum level at the surface of the semiconductor, i.e., h.nu. &gt; .epsilon..
The negative affinity III-V form of semiconductor photoemitter lowers the effective .epsilon. by the well known band-bending at the electron emitting surface of the semiconductor. The bending lowers the edges of the valence band and the conduction band at the semiconductor surface relative to the band edges in the bulk semiconductor, and effectively lowers the vacuum level relative to the conduction level, decreasing the height of the barrier to the electrons seeking to escape over the vacuum level.
Since the difference between the conduction band and the vacuum level is known as electron affinity, semiconductor devices employing band-bending wherein the vacuum level has effectively been made lower than the conduction band are referred to as negative-affinity devices.
In these negative affinity devices, it is also necessary that the photon energy h.nu. be greater than the band gap, i.e., the energy E.sub.G between the valence band and the conduction band, or EQU h.nu..gtoreq. E.sub.G
in order to obtain absorption of the photon and creation of the electron hole pairs. If the conduction level, while higher than the vacuum level so that no problem exists in having the electrons escape from the conduction level to the vacuum, is so high as to create a large bandgap, absorption of the photons and promotion of the electrons from the valence band to the conduction band does not take place and few electrons are emitted.
By proper selection of the III-V material, the bandgap E.sub.G can be lowered such that a profusion of electrons are promoted to the conduction band, but, as a result of the E.sub.G lowering, the conduction band falls below the vacuum level and the work function .epsilon. may prevent the electrons from escaping from the conduction band into the vacuum.
As a consequence of those conditions, a significant problem with electron emission from photoemitters such as III-V semiconductors is the work function .epsilon.. A considerable amount of work has been directed to lowering the work function of semiconductors; it has been shown that suitable surface activation with Cs.sub.2 O can substantially reduce the work function, to as low as 0.6 eV, i.e., corresponding approximately to photons of 2.mu. wavelength.
It would seem then that by selecting a III-V semiconductor with band bending and with a low E.sub.G, and by activating the surface with Cs.sub.2 O to reduce the work function, a very good long wavelength photoemitter should result. However, although such a device provides negative-affinity as desired, the junction between the III-V semiconductor and the other semiconductor material Cs.sub.2 O forms a large heterojunction barrier, and this interfacial barrier is higher than the vacuum level and the conduction band. This interfacial barrier height E.sub.B is typically about 1.15 eV and it prevents the desired long wavelength responsive photoemission. For a discussion of the interfacial barrier on III-V compounds see "Behavior of Cesium Oxide as a Low Work Function Coating" by J. Uebbing and L. James, Journal of Applied Physics, Vol. 41, No. 11, October 1970, pages 4505 to 4516, inclusive. Although some disagreement exists relative to the exact nature of this interfacial barrier as seen by reference to the articles "Long-Wavelength Photoemission From Ga.sub.1-x In.sub.x As Alloys" by D. G. Fisher et al, Applied Physics Letters, Vol. 18, No. 9, May 1, 1971, pages 371-373, "Interfacial Barrier Effects in III-V Photoemitters" by R. Bell et al, Applied Physics Letters, Vol. 19, No. 12, Dec. 15, 1971, pages 513-515, and "Photo-electron Surface Escape Probability of (Ga, In) As : Cs-O in the 0.9 to .about. 1.6.mu. m Range," Journal of Applied Physics, Vol. 43, No. 9, September 1972, pages 3815-3823, this barrier is clearly seen to prevent the escape of electrons from the conduction band to the vacuum, and to prevent attainment of efficient long wavelength infrared photoemission from III-V compounds.
The devices previously proposed for increased photoemission at long wavelengths involve multiple layer growths, uniform large-area operation of heterojunctions, biased layers, etc. leading to problems in surface and bulk nonuniformities in the grown layers, particularly where more than one heteroepitaxial layer is to be grown. In the case of approaches relying on tunneling through thin insulator layers, non-uniformity, already a serious problem with other than the simplest unbiased III-V negative affinity system, is a dominant and disabling phenomenon.